This contract is for the continued investigation of histocompatibility antigens unique to ethnic populations in the United States, with emphasis on Blacks and Native Americans. The contractor will screen sera from multiparous women to identify and acquire reagents capable of recognizing antigens unique to these populations. Even though much work has been done in this area over the past six years through previous contracts, there are still many individuals whose HLA typings are incomplete, placing them at a disadvantage when organ transplant is required. Full identification of the antigens of the MHC will lead to better matches for these populations, improving graft survival and reducing health care costs, much of which is federally subsidized.